In modern times the electrical energy storages are becoming an integral part of the distribution grids, ensuring the on-demand and reliable supply of electricity by the intermittent renewable energy sources and providing a stable and efficient operation of the base-load fossil-fuel fired and nuclear power plants around the clock.
Amongst the known methods for energy storage able to accumulate a lot of energy and store it over a long time-period, the proposed method for Liquid Air Energy Storage (LAES) (see FR Pat. No. 2,489,411) is distinguished by a much simpler permitting process and the freedom from any geographical, land and environmental constraints, inherent in other known methods for large-scale energy storage technologies, like Pumped Hydro Electric Storage (PHES) and Compressed Air Energy Storage (CAES). In the LAES systems liquid air is produced using a low-demand power from the grid, stored in the small volume tanks between the off-peak and on-peak hours and re-gasified and used as an effective working medium for producing a peaking power in the periods of high demand for power. However, producing a liquid air during off-peak hours is an energy intensive process, wherein a great deal of power is consumed from the grid by the electrically-driven atmospheric and recirculating air compressors. Therefore, many technical solutions have been proposed for reducing the energy consumption and losses in this process providing eventually an increase in the LAES round-trip efficiency (RTE).
The installation of one or two booster compressors downstream of the mentioned electrically-driven compressors is one of such solutions, which is long- and widely-used in the gas liquefaction technique. The booster compressors are driven by the work-performing turbo-expanders, wherein a major part of the air pressurized by said booster compressors is expanded and in consequence of this deeply cooled. A cold thermal energy of the expanded air stream is used for cooling and liquefaction of another, minor part of said pressurized air stream, whereupon said expanded air stream is recycled to the inlet of the electrically-driven compressor. The examples of gas liquefaction system with one booster compressor can be found in the U.S. Pat. Nos. 1,574,119, 5,836,173, et al., whereas the systems with two booster compressors are described in the U.S. Pat. Nos. 4,778,497, 4,894,076, 5,231,835, 6,484,533, et al. An increase in number of the installed booster compressors makes possible to somewhat reduce an electric power consumed by the compressor train, but this reduction does not provide a desirable cardinal increase in the LAES RTE. For example, at the same liquefaction capacity of 1.25 t/h a specific power consumption of the micro-scale gas liquefier with one booster compressor described in the U.S. Pat. No. 6,230,518 is equal to 800 kWh/t, whereas in the standard nitrogen liquefier with two booster compressors produced by the Cosmodyne LLC power consumption is reduced to 632 kWh/t only. Therefore, keeping a possibility for an in increase in number of the installed booster compressors, there is a need to find a new way for a more significant decrease in power consumed from the grid during charging the LAES.
One of such ways could be co-location of the LAES with the natural gas (NG) pressure reducing (city gate) station and recovery of an available energy of the high-pressure (HP) gas being presently wasted in the throttling valves. Replacement of the throttling valves by the turbo-expanders which convert a kinetic energy of the motive NG stream into useful grid power is a well-known technology described in many patents and patent applications. Most of the technical solutions of this kind are intended for generation of the mechanical or electrical power only. In these cases, a thermal energy at a rate of ˜3.5 kWth per each kW of additional mechanical power produced is usually consumed to provide the identical temperatures of the high-pressure (HP) and low-pressure (LP) gas streams at the inlet and outlet of the expander correspondingly. A thermal energy required for pre-heating the NG upstream of the expander is proposed to derive from combustion of a part of NG reduced in pressure (see e. c. U.S. Pat. Nos. 4,920,749 and 5,392,605), from the waste heat streams of co-located power or industrial facilities (see e. c. U.S. Pat. No. 5,425,230), or through converting a part of recovered electrical energy into a needed thermal one (see e. c. U.S. Pat. Appl. No. 2003/0070432). In so doing, there are not the known examples of converting the mechanical or electrical energy at the CG stations into a liquid air as an effective medium for storage of this energy and power from the grid during off-peak periods.
Recovering the mechanical or electrical energy at the CG stations may be supplemented by using a cold thermal energy of the LP NG stream outgoing from the expander for meeting the demands of the HVAC systems and commercial cold users. For these purposes, reduction in pressure of HP gas is performed with a moderate or without any pre-heating of this gas upstream of the expander. In the first case, an outlet temperature of LP gas is maintained at a level low enough for using this gas as a cold source in the air-conditioning systems, as it is described in the U.S. Pat. No. 7,272,932. In the second case, an outlet temperature of LP gas is maintained at a level significantly below 0° C. and may fall below a level of freezing the water components in gas stream. Therefore, a need for drying of HP NG stream upstream of the expander is determined with regard to a designed temperature of the LP gas and a water vapor content in HP gas. If the bulk or sole source of HP gas in the main pipeline is the NG from the LNG regasification terminal, the expansion of this gas may be performed without its pre-drying. Otherwise, lowering a temperature level of freezing the water components in gas stream or removal of H2O contaminants from it by the known means should be applied. The first approach is described in the U.S. Pat. No. 4,711,093, wherein methanol injection into HP gas stream upstream of the expander is used. However, several problems are inherent in this solution: an enhanced complexity of design and operation, consumption of fuel for regeneration of methanol from the aqueous methanol condensate and for heating of LP gas after recovery of its cold thermal energy, losses of methanol and NG from the system, and limitation of a bottom temperature level of cold thermal energy by −65÷−75° C. The second approach seems to be more suitable, since it makes possible to further lower a temperature level of the cold thermal energy of LP gas. However, the examples of using this cold in the LAES systems are unknown.
By and large the conducted analysis of the known technologies for recovering a waste energy of NG stream at the CG stations has not revealed any technical solutions using an extracted mechanical or electric power and cold for storing a power from the grid in general and for production of liquid air as a storable medium during off-peak hours in particular. The invented method offers such solution through a profitable integration of the LAES and CG stations; in doing so recovery of kinetic energy of the HP gas and cold thermal energy of the deeply cooled LP gas at the CC station provides a significant decrease in power consumed from the grid during charging the LAES facility. Here “deeply cooled LP natural gas” or “deeply cooled air” are to be understood to mean a gas or an air cooled down to and below −100° C.